Ribulose 1,5-bisphosphate (RuBP) carboxylase catalyzes the primary assimilation of carbon dioxide. Based on prior active-site and effector-site modification studies, we intend to: a) obtain direct chemical evidence for the distinctness of the effector site(s) and the catalytic site. Peptides covalently labeled with pyridoxal phosphate, under both activation and inactivation conditions will be purified and sequenced. b) continue active site-modification studies with several site-specific ligands. Additional information about the environment of the active site will be obtained upon isolation of active-site residue containing peptides. c) determine the stoichiometry of Co2 ion and CO2 labeling of spinach RuBP carboxylase and determine the stoichiometry of substrate and effector binding to Co(III) complexes of spinach and R. rubrum carboxylase. d) determine the function of the small subunit, using the Form I and Form II carboxylases isolated from R.sphaeroides as the model system.